Currently, via a Chrome browser session, you can navigate to Stadia Settings by clicking on your avatar, and there is an option available within those settings called "Redeem code", selecting that gives you the ability to enter a code.

There was an option before in the Stadia App to redeem codes, was pretty sure, it was available when clicking on your profile in the top right corner. But it looks like they removed it with the last update for the Stadia App, so I can't find that option anymore.

At the moment there are no free games via code to claim, you can only claim the Stadia Pro games for free or the Buddy Pass (if you got one from a founder), so at the moment entering a code there is not necessary, but yeah...maybe they implement it in the future again?

Thanks for your reply. I'm a game reviewer, so I actually have the codes and have been beating my head against a wall trying to figure out how to enter them. I was on the phone for over an hour yesterday with Stadia support, and they couldn't tell me as much as you just did - so thank you!

Currently, via a Chrome browser session, you can navigate to Stadia Settings by clicking on your avatar, and there is an option available within those settings called "Redeem code", selecting that gives you the ability to enter a code.